1. Field of the Invention
This invention pertains to mooring systems for ships and other floating structures. More particularly, it pertains to an essentially permanent single-point mooring system for use with floating structures intended to be moored in a given location for long periods, say several years.
2. Review of the Prior Art
Increasing attention is being given to marine transportation of liquefied natural gas (LNG), in addition to petroleum, to meeting increasing and increasingly critical energy requirements worldwide. Several vessels for ocean transport of LNG have already been built, and more are now under construction or in stages of planning. As LNG liquefaction and gasification facilities now exist only at land-based locations, it will be necessary for these LNG tanker ships to load and unload cargo at existing ports. These facilities are usually located in the ports themselves, often some distance from the natural gas source which may be at an offshore location.
Ocean ports usually develop into substantial centers of population. It is known that, even with the practice of stringent safety precautions, LNG liquefaction and gasification facilities present hazard, more or less, to adjacent structures and populations in the event of an accident. It is therefore desirable, subject to compatibility with marine transport cargo transfer requirements, to locate LNG liquefaction and gasification facilities remote from existing population centers; this is possible only with new ocean ports, if they can be developed, and only until such time as the new ports themselves become population centers.
In many areas, the principal sources of natural gas are in offshore gas fields, such as in Indonesia and the Persian Gulf, for example. To supply LNG from these areas, it is presently necessary to pipe the gas from the offshore fields to onshore gas liquefaction plants from which the LNG is transported to the LNG tanker ships.
To minimize the hazards of LNG treatment facilities to adjacent population centers, and to reduce the cost of producing LNG, it has been proposed to locate LNG liquefaction facilities at offshore locations, particularly in or close to offshore gas fields. It has also been proposed to locate LNG gasification facilities at offshore locations safely spaced from, but otherwise reasonably close to centers of gas consumption or to connection to gas transport pipelines. Such facilities have been proposed in the context of permanent structures erected on the ocean floor, and in the context of floating factory ship vessels.
The disadvantage of permanent structures erected on the ocean floor is that they usually are contemplated remote from major manufacturing centers which means that they are very costly to construct, and that they require considerable time to construct which means that they are subject to delay and damage in construction because of storms, high seas and the like. The advantage of such LNG treatment facilities on floating structures is that the structures, such as factory ships, can be built efficiently and economically in existing centers of commerce, such as shipyards, and towed or self-propelled to their intended location of use. The difficulty, at least to date, with floating LNG treatment facilities has been in the area of mooring the facility in place for long periods in such a way as to enable the floating structure to conform to, rather than fight against the variable forces of wind and wave action. Another difficulty has been that of supplying gas to or from a floating LNG factory ship with safety over a long period.
Various approaches to offshore mooring of ships have been proposed. The proposals pertinent to mooring of tankers at offshore locations while taking on and offloading cargo (petroleum or petroleum distillates) are not adaptable directly to LNG factory ships; these proposals pertain to short-term mooring of the vessel, not to a mooring which may exist for several years. Other proposals pertinent to long-term mooring arrangements have been made in the context of factory ships or other vessels moored in harbors or other protected waters where the moored vessel is not exposed to the full effects of wind and wave action which may come from any compass direction. These latter proposals, therefore, are not directly adaptable to long-term mooring of vessels at fixed locations in places significantly offshore, as in the open ocean.
For example, a proposal has been made for a permanent mooring for an offshore processing and storage facility. In this proposal, the mooring is totally resilient. The mooring and the oil supply line it supports flex and move in response to mooring loads. In this proposal, there are flexible oil line elements (hoses and swivels) and mechanical oil line elements (valves and other closures, for example) below the water which require maintenance and/or periodic replacement. To facilitate maintenance and replacement of these elements, the oil lines are redundant. Perhaps most significant is the fact that the oil pressure may be in the range of 100 to 200 psig, whereas natural gas line pressure may be in the range of from 600 to 1200 psig. As compared to natural gas, the lower oil pressure encountered in this proposal has two effects. The considerably lower oil supply line pressure produces much lower stresses in the supply piping. The lower pressure and the higher viscosity of oil means that initial leaks will be small and can be dealt with before becoming catastrophic. Oil, if light enough, will float on the water surface in such manner to make difficult ignition of the oil. Natural gas leaking from submerged supply lines will expand in volume as it rises to the water surface, and will rise above the water surface where it can be ignited very easily. Thus, natural gas provides an entirely different problem from oil. The moorings useful with offshore oil treatment and transfer facilities can be used with natural gas only with considerable difficulty and hazard.
A need is thus seen to exist for a system for safely mooring floating vessels at open-ocean offshore locations for very long periods, in the order of several years. Such a system, to be well suited to safe supply of gas to and from the vessel where the vessel is an LNG factory ship, should include a single mooring point which is essentially fixed relative to the ocean floor. The mooring system should impose minimum constraints on the vessel relative to the mooring point so that the vessel can move to the greatest extent possible in response to wind and wave action, to thereby minimize the loads imposed on the vessel and the mooring by wind and wave action. On the other hand, the mooring system must be able to withstand and preferably absorb those loads, and the motions related to such loads, which are necessary to keep the vessel connected to the mooring point. Ideally, the mooring system should be relatively quickly connected in the first instance, and it should be arranged so that in an emergency the coupling between the vessel and the mooring point can be severed to enable the vessel to be moved away from the connection point.